1. Field of the Invention
This invention relates to an aluminum base alloy containing boron having a neutron absorbing capacity which is effective for a structural material for a transporting packaging (cask) for transporting spent nuclear fuel or a storage cask container and the like, and its manufacturing method.
2. Description of the Related Art
As the structural material having a neutron absorbing capacity, alloy added with boron within aluminum alloy is used. In order to manufacture such an alloy as described above, boron in the form of powder is normally added in molten pool of Al, (Al-bath) or boron in the form of boronfluoride such as KBF.sub.4 is added in Al-bath so as to generate intermetallic compound, although various kinds of improvements have been applied in order to increase mechanical properties such as strength or ductility.
For example, in the gazette of Japanese Patent Laid-Open No. Hei 1-312044 is disclosed a method in which boron is added in the form of powder of 12 aluminum boride (AlB.sub.12) or in the form of Al--B mother alloy mainly containing at least AlB.sub.12 so as to restrict a reaction between B and Mg and then its strength is reduced on the basis of production of these intermetallic compounds. In addition, in the gazette of Japanese Patent Laid-Open No.Hei 1-312043 is disclosed a method in which a melting processing is carried out in a high temperature region of 1200.degree. C. or more in order to restrict a reaction between B and Mg. Further, in the gazette of Japanese Patent Laid-Open No.Hei 4-333542 is disclosed a method for getting Al--B alloy having a low viscosity and having a superior castability by reacting KBF.sub.4 with Al within a temperature range of about 680 to 850.degree. C. and adding a small amount of K.sub.2 TiF.sub.6 in the melts of Al--B alloy containing the generated AlB.sub.2 crystal in order to eliminate a high viscosity of the melts and improve a forming and workability.
However, all these alloys manufactured by these methods show a problem that once they are solidified, they may not be utilized again. That is, when the scrap alloy is melted again, an intermetallic compound AlB.sub.12 which is quite stable and fragile is enevitably produced, so that there is a problem in view of recycling of it as the structural material. Although it is necessary to apply hot-rolling or extrusion of the scrap alloy in order to reproduce it as the structural material, an existence of AlB.sub.12 within an entire boron compound with a volumetric rate of 20% or more causes its working to be quite difficult and this fact becomes a cause for recycling of the scrap alloy difficult.
The alloy described in the gazette of Japanese Patent Laid-Open No.Hei 4-333542 in the aforesaid methods is an Al--B alloy in which it contains 0.001 to 0.05 wt % of Ti and an entire boron is composed of AlB.sub.2 crystals, although this alloy does not contain Mg, Si and the like, so that this alloy has disadvantages that it is inferior in its mechanical properties such as strength or the like, its remelting causes AlB.sub.12 to be enevitably produced and then the scrap alloy may not be utilized again.
In addition, all the aforesaid methods apply natural boron. Originally, boron has a isotope composition composed of .sup.10 B (about 20%) and .sup.11 B (about 80%), and boron having a superior neutron absorbing capacity is mainly .sup.10 B. Due to this fact, in order to get a desired neutron absorbing capacity, it is actually necessary to use a large amount of expensive boron and so it is accompanied with a problem of increasing a manufacturing cost.